Functional analysis of tomato CHIP ubiquitin E3 ligase in heat tolerance

Plants have evolved genetic and physiological mechanisms to mitigate the adverse effects of high temperature. CARBOXYL TERMINUS OF THE HSC70-INTERACTING PROTEINS (CHIP) is a conserved chaperone-dependent ubiquitin E3 ligase that targets misfolded proteins. Here, we report functional analysis of the SlCHIP gene from tomato (Solanum lycopersicum) in heat tolerance. SlCHIP encodes a CHIP protein with three tandem tetracopeptide repeat (TPR) motifs and a C-terminal U box domain. Phylogenetic analysis of CHIP homologs from animals, spore-bearing and seed plants revealed a tree topology similar to the evolutionary tree of the organisms. Expression of SlCHIP was induced under high temperature and was also responsive to plant stress hormones. Silencing of SlCHIP in tomato reduced heat tolerance based on increased heat stress symptoms, reduced photosynthetic activity, elevated electrolyte leakage and accumulation of insoluble protein aggregates. The accumulated protein aggregates in SlCHIP-silenced plants were still highly ubiquitinated, suggesting involvement of other E3 ligases in ubiquitination. SlCHIP restored the heat tolerance of Arabidopsis chip mutant to the wild type levels. These results indicate that tomato SlCHIP plays a critical role in heat stress responses most likely by targeting degradation of misfolded proteins that are generated during heat stress.